1. Field of the Invention
The present invention relates to a circuit substrate provided with a plurality of heat generating elements having heat generating resistors, and a producing method therefor. In particular, it relates to a circuit substrate for a liquid discharge apparatus, provided in a liquid discharge head such as an ink jet head for converting an electrical energy into thermal energy by a heat generating resistor and utilizing such thermal energy for discharging liquid, and a producing method therefor.
2. Related Background Art
In the following, there will be explained a prior method for producing a circuit substrate, taking an ink jet head as an example.
An ink jet recording apparatus discharges ink as a minute liquid droplet from a discharge port onto a recording material, thereby recording a high definition image. In this operation, the ink jet recording apparatus converts an electrical energy by a heat generating resistive member into thermal energy, and generates a bubble by such thermal energy. By an action force of the bubble, a liquid droplet is discharged from a discharge port at a front end of a liquid discharge head. The liquid droplet discharged from the discharge port is deposited on a recording material to record an image. In general, such liquid discharge head includes a circuit substrate provided with a plurality of heat generating resistive members for converting electrical energy into thermal energy.
The heat generating resistive member is an electrothermal converting member for converting electrical energy into thermal energy. The heat generating resistive member is protected by an upper protective layer from contact with the ink.
More specifically, a resistive material layer and an electrode material layer are formed on an insulating surface, a part of the electrode material layer is removed to form a pair of electrodes, between which the resistive material layer constitutes a heat generating portion. Then there are formed a protective layer for protecting these layers from the ink, and an anticavitation layer for protecting the protective layer from a chemical or physical damage resulting from heat generation.
In such configuration, there may result an unsatisfactory step coverage in edge portions of the electrodes for defining the heat generating portion. Such situation is schematically illustrated in FIG. 7. An illustrated circuit substrate has an oxide layer 61, on which a resistive layer 62 such as of TaSiN is formed. The oxide layer 61 is formed by oxidizing a surface of a Si substrate, and FIG. 7 illustrates only the oxide layer on the surface. On the resistive layer 62, an Al wiring 63 is provided but is partially absent on the resistive layer 62, and the resistive layer constitutes a heat generating portion 64 in a portion where the wiring 63 is absent. On the resistive layer 62 and the wiring 63, there is provided a protective layer 65 formed, for example, by P—SiN for protecting these layers from the ink, and, in the heat generating portion 64 of the resistive layer, an anti-cavitation layer 66 formed by Ta is provided on the protective layer 65 for protecting the protective layer 65 from a chemical or physical damage resulting from heat generation. An edge portion of the wiring 63 is illustrated with a substantially vertical structure.
The circuit substrate for a liquid discharge apparatus is provided, at a high density, with a plurality of such heat generating elements including heat generating resistive members as explained above, thereby enabling a high definition image recording. Each heat generating element is serially connected with a power transistor (not shown) for on-off controlling a current in the heat generating resistive member. Also discharge ports are formed on the circuit substrate to constitute a liquid discharge apparatus.
In FIG. 7, in portions indicated by arrows, P—SiN and Ta are unable to cover the wiring 63 with satisfactory step coverage.
In order to resolve such difficulty, there has been developed, as described in U.S. Pat. No. 5,376,231, a method of giving a tapered shape to the edge portions of a pair of electrodes of a heat generating element. Such method allows to improve the coverage of the protective layer and the anti-cavitation layer also in the edge portions of the paired electrodes of the heat generating element. In the following, there will be explained a method of producing a circuit substrate, including a method of forming tapered portions in the paired electrodes of the heat generating element.
FIG. 6 is a flow chart showing a process flow for preparing a prior heat generating resistive member, and FIG. 5A is a cross-sectional view of a heat generating resistive portion after the preparation. At first a Si wafer is used as a substrate, bearing a SiO2 oxide layer 41 of a thickness of several micrometers by a thermal oxidation process. A resistive material layer of TaSiN or the like of a thickness of about 50 nm is formed by sputtering thereon. Thereafter, for example, an Al film is formed with a film thickness of about 200 nm to form a wiring material layer. Then a resist layer (1) is formed and patterned, and the Al and the resistive material layer are dry etched, for example, by RIE to achieve element isolation, thereby forming a wiring layer 43 and a resistive layer 42. After the resist layer (I) is eliminated by O2 ashing, a resist layer (II) is formed and patterned, and the Al is eliminated by wet etching from a portion of the resistive layer to constitute a heat generating portion 44. In this operation, as the wet etching solution, there is employed an organic alkali etching solution containing tetramethyl ammonium hydroxide (hereinafter represented as TMAH) as a principal component or an acid etching solution containing phosphoric acid as a principal component, whereby an end portion of the resist layer (II) is retracted by etching during etching of Al, thereby forming a tapered shape in the edge portions of a pair of electrodes to be formed. Then a SiN protective layer 45 of a thickness of about 300 nm is formed by plasma CVD, and a Ta film 46 is formed by sputtering. An unnecessary portion of the Ta film 46 is removed by dry etching to obtain a circuit substrate.
However, with the recent progress in the printing technology toward a higher definition, an ink discharge amount per discharge has decreased from several tens of picoliters to several picoliters. Also there is a strong demand for an even higher printing speed, and a longer dimension of the substrae is desired for reducing the number of reciprocating motions of the liquid discharge head. For these reasons, the number of heat generating elements per circuit substrate is increasing from several hundreds to several thousands. Under such situation, even the aforementioned method of forming a pair of tapered electrodes shows a slight fluctuation in such tapered shape, thus deteriorating the coverage of the protective layer and the anti-cavitation film in a part of the heat generating elements.
Therefore, an object of the present invention is to provide a circuit substrate for a liquid discharge head, showing a satisfactory coverage of the protective layer and the anti-cavitation film on the heat generating elements and providing an excellent durability, and a producing method therefor.